It is common practice to inspect work pieces subsequent to production on a coordinate positioning apparatus, such as a coordinate measuring machine (CMM), in order to check for correctness of predefined object parameters, like dimensions and shape of the object. Moreover, a detection of a surface of an unknown object is of interest in many industrial applications. Such measurement typically also may be provided using a coordinate measuring machine or any other suitable type of scanning device.
In a conventional 3-D coordinate measurement machine, a probe head is supported for movement along three mutually perpendicular axes (in directions X, Y and Z). Thereby, the probe head can be guided to any arbitrary point in space of a measuring volume of the coordinate measuring machine and the object is measurable with a measurement sensor (probe or probing unit) carried by the probe head. Such probing unit can be designed as a tactile probe or an optical sensor providing measurements of surfaces e.g. based on the principle of triangulation or interferometry.
In a simple form of the machine a suitable transducer or linear encoder mounted parallel to each axis is able to determine the position of the probe head relative to a base of the machine and, therefore, to determine the coordinates of measurement points on the object being illuminated by the sensor. For providing movability of the probe head a typical coordinate measuring machine may comprise a frame structure on which the probe head is arranged and driving means for moving frame components of the frame structure relative to each other.
Typical generic systems are coordinate measuring machines of the Portal type such as are described, for example, in DE 43 25 337, or 3D coordinate measuring articulated arms that, for example, are known from U.S. Pat. No. 5,402,582 or EP 1 474 650.
An advantage of using an optical sensor can be that it does not have to be in contact with the part to be measured and therefore does not deform the part during the measurement or damage it, as may be the case with a tactile probe.
However, it goes hand in hand with the optical measurement methods for coordinate measuring machines that there is a need to guide an optical signal from a sensor element to a control unit of the coordinate measuring machine, wherein typically here is used an optical fibre as optical conductor and for optical signal transmission of the measuring radiation.
Suitable optical fibres are widespread in telecommunications for signaling transmission. In order to couple said optical fibres, a multiplicity of plug-in couplings exists that are coordinated with the respective application purposes, and enable even high energy optical radiation to be transmitted via the optical fibers and the plug-in couplings in conjunction with low losses. However, the plug-in couplings have a decisive disadvantage: they have mainly been produced for static connections. Because of the high level of sensitivity of the optical interface to contamination and damage, they cannot be frequently plugged in. The forecast service life of known couplings is e.g. 500 to 1000 plug-in cycles. However, in practice they are opened only to be serviced. In addition, in order to ensure optimum transmission, the cleaning of the surfaces of the optical fibers at the optical interface is required before plugging together is undertaken anew.
In order to be able to use a coordinate measuring machine to measure complex measuring objects, for example engine blocks, there is a need to change the sensor element relatively frequently. The type of optical probe used for measuring a particular work piece has to be chosen in such a way that measuring properties of the probe fit to the shape or to topographic properties of respective parts of the work piece.
For providing individually suitable probes fitting to respective measuring requirements frequent changing of the probe attached to the probe head of the CMM is typically necessary. Moreover, a number of specified probes for respective measuring requirements are to be available for guarantee precise measurement of different work pieces.
For instance, for measuring a borehole the probe may preferably comprise a prism or mirror to emit the measuring beam in a predefined angle, e.g. 90°, relative to an extension axis of the probe stylus.
As a rough estimate, there is a need for a sensor element to be changed approximately once per hour. Since the coordinate measuring machines often are driven round the hour, 100 plug-in cycles are easily reached per week, and so the optical plug-in couplings known from telecommunications would reach the end of their expected service life as early as after approximately 3 months.
In order for it to be possible to make rational use of optical sensor elements in coordinate measuring machines, the optical couplings must fulfill at least the same requirements with regard to robustness and precision as do mechanical or mechanical/electrical coupling elements.
A coordinate measuring machine and a respective probe thus typically comprise an interface which allows to both mechanical and optical, in particular also electrical, coupling of the two components. By such interface the probe can comparatively quick and with low effort be attached to the probe head and carried and positioned by the probe head for measuring purpose. Such an optical-mechanical interface is for instance known from EP 2 356 401 B1.
According to that approach the ferrules or fibres have to be precisely aligned each time the elements are coupled to one another. The couplings comprise specific alignment units for that. Such system provides for quite tight tolerances for angular alignment which simultaneously is one of the disadvantages of that approach.